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Re: More questions on the Burma microplate (allegedly where the quake occurred)



Can y'all please tell me what are the exact boundaries of the Burma
microplate?   Do the boundaries other than the trench consist of fault lines
or trenches or some recognizable geographic feature?

Something else I'd particularly like to understand is the motion of the
Burma microplate.

Kerry Sieh, the adviser of the person who wrote the dissertation that mapped
the faults off of Sumatra and that I found so informative on the geology of
the area, is actually director of the California Earthquake Center, and has
made some comments to the press and a press release about the quake.

He said that the quake was caused by a 700 mile or so long piece of the
Burma microplate jumping up onto the Indian plate.  This caused that part of
the sea floor to rise, and caused the land behind the new thrust fault to
drop, which dropped parts of northern Sumatra below sea level and left them
permanently swamped, particularly the capital of the state of Aceh.   He
doesn't mention any north/ south motion along slip/ strike faults like the
San Andreas fault, such as NEIS mentions, and an article that primarily
cited Kerry Sieh insists happened.

On the other hand, the motion of the India plate is northeast, the plates
meet at an oblique angle, and the entire function of the umpteen slip/
strike faults in the area includnig the West Andaman Fault is to perform the
north-south part of the motion.   Aftershocks of thrust-fault quakes in the
area usually involve compensating north-south motion along the slip/strike
faults.

So has there been motion on the slip/strike faults, and if so, where?

None of the other work I've read by Prof. Seis' group mentions the Burma
plate by that name, despite mapping in detail the tectonics of the Sumatran
portion of the subduction zone.    And in his comments to the media he
didn't define the Burma microplate at all.   So I really don't know what the
quake did or affected.

The Mark Hall article mentions the BUrma plate several times, and also
mentions an arc sliver that some geologists on the geology newsgroups tell
me is more than likely the same thing - but he never defines it.

I'm looking at an abstract of a paper that says the Menawai Fault and the
Sumatra fault system take up the functions of the West Andaman Fault - which
I already knew -  my guess would have been the Mentawai Fault and the West
Andaman Fault bound the Burma plate, but you're telling me the Burma plate
includes teh island of Sumatra, which would mean that not even the Sumatra
fault forms part of its boundary.

Sieh says the entire Burma plate jumped and not just a strip of it; the
strip that jumped up left the island of Sumatra LOWER than it originally was
behind it, and the strip of land that jumped up is said to be somewhere
between 9 and 100 miles wide - not wide enough to encompass Sumatra.

I wrote to Kerry Sieh and also to the author fo that dissertation, who still
works for Kerry Sieh - but I guess that this week they are in Sumatra trying
to fill in an apparent hole in their data on relationships between the
growth of coral reefs off of Sumatra and its seismology.    They had found a
relationship but may not have examined the specific area where the quake
occurred and had not been able to get back there until now.

They specifically found that the sea bottom rises steadily prior to a quake
that causes it to abruptly drop, and drops steadily prior to a quake that
causes it to abruptly rise.    The dissertation reports among other things
on a inexplicable uplifting of the sea floor under two islands several
islands south of where the quake occurred - and apparently the southern
boundary of the section that moved was about where teh epicenter was - so
the quake that happened didn't cause the part of the sea floor that they
looked at and found had uplifted to drop back down.    Part of what was so
inexplicable is exactly why the quake was so surprising - that portion of
the fault has been completely quiet.      Now he's saying to the media that
he's concerned that part of the plates are still locked and there will be
another quake to the south of it.

Like all articles on the tectonics of the Andaman Sea and Sumatra area I
have found that are clearly written and that go into any detail, Curray's
paper that I"m looking at doesn't mention a Burma plate - but it has an
e-mail address attached to it, and  I will write and see if he knows what
the term refers to as well as Kerry Sieh apparently does.

I'm starting to wonder if the term "burma plate" could be some kind of new
age jargon!    I've not seen the term used very precisely anywhere, noone
who uses the term is saying in precise terms what it means, even in answer
to my questions, and noone who has communicated clearly and precisely about
the tectonics of the region has used the term when they were trying to
communicate clearly and precisely about the tectonics of the area in
scientific papers - though apparently Kerry Sieh is willing to use the term
with the media!

Yours,
Dora Smith
Austin, Texas
villandra@austin.rr.com
----- Original Message -----
From: "Gautam Majumdar" <gmajumdar@freeuk.com>
To: <david.marjanovic@gmx.at>
Cc: "DML" <Dinosaur@usc.edu>
Sent: Monday, January 03, 2005 6:24 AM
Subject: Re: More questions on the Burma microplate (allegedly where the
quake occurred)


> On Sunday 02 Jan 2005 16:34, David Marjanovic wrote:
> > > Probably to avoid confusion with Andaman basin which covers only
> > > part of the Burma microplate.
> >
> > What about the Andaman islands, which are entirely on the plate? Or
> > the Nicobar islands (ditto)?
> >
> Yes. Both groups of islands are entirely on the Burma microplate. As
> Burma has changed its name back to an earlier one - Myanmar - they may
> ask the name of the microplate to be changed as well :-)
>
> --
>
> Gautam Majumdar
>
> Please send e-mails to gmajumdar@freeuk.com